The process of withdrawing spent or partially spent cooking fluid at the cooking temperature, this fluid being known as “black liquor”, from various positions in the digester with a certain level of residual alkali, and the subsequent leading of this spent cooking fluid, from which the pressure has been partially or fully released, at a temperature of approximately 90-120° C. to an impregnation vessel for impregnation, during the continuous cooking of chemical cellulose pulp in a two-vessel system is known. This process is known as “black liquor impregnation”. The principal aim of this type of impregnation is to obtain a good impregnation of the alkali black liquor, such that the chips are fully neutralised and given an alkali pH. A further aim is to be able to conserve to a greater degree the heat in the withdrawn cooking fluid in order to heat the colder chips in the impregnation vessel.
The conservation of a part of the heat from the hot black liquor in association with black liquor impregnation is also known. This traditionally takes place through the pressure of the steam being removed with pressure-reduction cyclones, and where this flash steam is used for, among other purposes, the pre-treatment of the chips, or for other heating purposes. The conservation of the heat energy from the black liquor by allowing the black liquor to pass through a heat exchanger is also known.
SE 518 957 C2 reveals a method for improving the heat economy of a continuous digestion system. Hot black liquor is withdrawn in this case from the digester, and returned to the bottom of the impregnation vessel, with the aim of increasing the temperature of the chips in the feed line up to the digester. A portion of the heated fluid is withdrawn from the feed line at the top separator and sent to the impregnation vessel in order to function as impregnation fluid.
SE 518 738 C2 reveals a method and an arrangement for improving the impregnation of chips in a continuous digestion system. Chips that have not been pre-treated with steam are fed to an impregnation vessel in which a fluid level (LIQ_LEV) is established that lies under the highest level of the chips (CH_LEV).
An improved impregnation arrangement for the chips is achieved through the addition of impregnation fluids (BL1/BL2/BL3), in the form of cooking fluids that have had the full cooking temperature, with increasing temperatures at different positions (P1, P2, P3), and through the establishment of a zone (Z1) of counter-current flow at the highest part of the impregnation vessel.
The requirement for steam pre-treatment can in this way be considerably reduced, while the amount of expelled weak gases is at the same time reduced to a minimum. A major part of the readily volatile compounds in the wood is bound in the withdrawn impregnation fluid (REC).
U.S. Pat. No. 5,679,217 reveals an impregnation method in which transfer fluid is withdrawn from the top separator of the digester. Furthermore, black liquor is withdrawn from the digester through a withdrawal strainer (8). The fluid withdrawn from the top separator and the black liquor are mixed in a line (11) and returned to the impregnation vessel. A portion of the mixture in the line (11) is led to the start of the impregnation vessel in order to there function as impregnation fluid. A second part of the mixture in the line (11) is heated and subsequently led to the bottom of the impregnation vessel in order to pre-heat the chips and to function as transfer fluid. The aim of this impregnation method is to optimise the fluid/wood ratio during the cooking process.
SE 527 058 reveals a method in which the chips after impregnation in an impregnation vessel are fed together with circulation fluid in a feed line to a top separator on a subsequent digester. A portion of the circulation fluid is withdrawn at the top separator and returned to the bottom of the impregnation vessel through a return line. Black liquor is withdrawn from the digester and passes to the impregnation vessel through a black liquor line. A heat exchanger is arranged between the return line and the black liquor line. The heat exchanger allows the transfer of heat between the two lines without allowing the exchange of fluids. The temperature of the black liquor has been reduced after its passage, while the temperature of the return line has at the same time been increased. Black liquor impregnation is achieved at a lower temperature with the invention, while the chips are at the same time heated at the bottom of the impregnation vessel, and this means that the need for heating by steam at the top of the digester is significantly reduced.
Characteristic of all of the technologies for impregnation described above is that the impregnation fluids that are used for the impregnation are directly adapted to be suitable for the digestion process that is used at the digestion plant, since the impregnation fluids that are used are fully, or partially, constituted by cooking fluid withdrawn from a subsequent digester.
A first object of the present invention is to achieve a method for the impregnation of chips in a continuous digestion system, where the impregnation method can be simply adapted for different cooking technologies such as, for example, LO-Solids, MCC and EMCC, where the impregnation can be carried out independently of the digestion process used.
A second object is to achieve an impregnation method in which the impregnation fluid is not constituted by a cooking fluid that has been withdrawn from a digester, but is constituted solely by the flows of the impregnation vessel itself, with the addition of white liquor.
A third object of the present invention is to obtain an impregnation that can be easily implemented in different cooking processes, by using the fluid flows that are already present in the digestion plant.
A fourth object is to avoid using black liquor withdrawn from the digester as impregnation fluid.
A fifth object is to use the heat energy from black liquor that is to be sent to the recovery process in order to heat the impregnation fluid.